Generally, it is desirable in industrial operations to inhibit the formation of deposits. For example, one of the major difficulties posed by water injection in oil fields, i.e., the physicochemical incompatibility between the injected waters and the field waters, results in the formation of deposits, for example alkaline earth deposits, producing irreversible degradation particularly detrimental to the reservoir rock and to the bottom and the surface production equipment.
Many anionic polymers and copolymers have been used as agents for inhibiting inorganic deposits, as disclosed for example, in U.S. Pat. Nos. 4,072,607, 4,175,100; and 4,518,511. Generally, their weight distribution ranges from 500 to 25,000. Some of them may have a weight distribution in the range of from 3,000 to 300,000, which is controlled, as disclosed in U.S. Pat. No. 4,143,222.
For example, U.S. Pat. No. 4,072,607 discloses the preparation of a acrylate-acrylamide copolymer mixture in molar proportions from about 20:1 to 1:1 having a molecular weight ranging from 500 to 12,000, with a distribution that such at least 60% of said copolymer has a molecular weight of from 500 to 2,000, at least 10% a molecular weight of from 4,000 to 12,000 and the remainder has a molecular weight of from 2,000 to 4,000, these distributions being determined by exclusion chromatography.
This copolymer is prepared by polymerizing acrylamide monomer in the presence of an initiator and a chain transfer agent, the obtained polyacrylamide being partially hydrolyzed to sodium acrylate. With a chain transfer agent introduced during polymerization, first in a proportion of 3% and then in a proportion of 16% with respect to acrylamide, the above-mentioned weight distribution is observed.
Moreover, U.S. Pat. Nos. 3,665,035 and 3,756,257 disclose the use, as a crystallization prevention agent in the treatment of oil wells, of a polymer composition obtained by reacting at 24.degree. C., in an inert solvent such as water, 80-90 parts by weight of acrylic acid, as single monomer, with 7-15 parts by weight of thioglycolic acid, as a chain transfer agent, and 1.5-5 parts by weight of ammonium persulfate, as a radical initiator. The acrylic acid obtained has an average molecular weight of from 500 to 1,000.
It is prepared by a process comprising one or more stages, with one or two additions to the inert medium formed by the acrylic acid polymer obtained at the end of the first stage, of a new charge of acrylic acid, chain transfer agent and radical initiator catalyst, which will polymerize and form an additional polymerization product and so on.
It has been discovered that this type of polymer is only partially adapted to the treatment of deposits formed of complex sulfates of alkaline-earth metals under severe thermodynamic conditions.
On the other hand, it has been discovered that copolymers of high molecular weight are difficult to solubize and hence difficult to use in the prevailing conditions of deep fields with highly salted water (e.g., 150-350 g/l salt content), high temperature and pressure (e.g., 120.degree.-150.degree. C., 100-400 bars) and had a lower inhibiting activity in the above-mentioned thermodynamic conditions. Moreover, it has been observed that the inhibition effect of these products was very short (a few hours) even under thermodynamic conditions of low severity.
It has also been discovered that these products of the prior art were not soluble to a substantial extent in water having a salt content of 30-100 g/l and, accordingly, that they were not useful when sea water, for example, is injected.